1. Field of the Invention
The present invention relates to a water sensing wire for a power cable and a power cable using such a water sensing wire. Power cables contemplated by the invention relate generally to the high and extra high voltage range of 40 to 500 kV. However, the present invention may also be applied to power cables in the medium and low voltage range such as 500 V to 40 kV or telecommunications cables.
In power cables water sensing wires are generally used in order to detect a water intrusion into the power cable, something which presents a critical condition for the power cable mechanically as well as electrically. Due to the construction of the power cable the water sensing wire is exposed to various kinds of mechanical influences during the manufacturing of the power cable, during the installment of the power cable in a system and even after many years due to environmental influences such as temperature, vibrations etc., substantially shortening the lifetime of the water sensing wire or substantially decreasing the mechanical and/or electrical properties of the water sensing wire.
The present invention in particular aims at providing a water sensing wire and a power cable having an increased lifetime.
2. Description of the Related Art
FIG. 4 shows a typical example of a conventional power cable PCA comprising one or more water sensing wires WSW. As shown in FIG. 4, the power cable PCA is typically composed of a core consisting of the central conductor PC and an insulation layer PI over which a bedding PBE, the cable screen PSC and the outer sheath PSH are arranged in the stated sequence. Typically, the conductor PC is made of copper, the insulation PI is made of polyethylene, the bedding PBE is made of some kind of semiconductor fabric wrapped around the core PC, PI whilst the screen PSC consists of a plurality of wires, as will be explained below. Thus, the high power cable PCA is surrounded by an insulating and water proof sheath PSH and in many cases this sheath PSH consists even of a double layer of a metal or metal foil layer in combination with an outer layer of plastic (layered sheath).
With such a construction it is hoped that intrusion of water—also in form of water vapor—into the cable core PC, PI can be prevented. The water intrusion leading to so-called “water treeing effects” must be prevented because water accelerates aging processes in the insulation materials of the insulation PI (and the outer sheath PSH) made of cross-linked polyethylene. Therefore, such water intrusion leads to an early failure of the cable.
However, since it cannot be guaranteed that no water intrusion occurs, more recently power cable technology has employed a water intrusion sensing mechanism to be able to at least detect the water intrusion if such water intrusion does occur. Thus, a shutdown of the power cable system or the adjustment of electrical conditions can be performed. Such water intrusion detection mechanisms are also capable of locating the exact position of the water intrusion such that a portion of the cable can be cut out and replaced by a new one if a water intrusion failure occurs.
For this purpose the power cable is equipped with one or more water sensing wires WSW, which are, as shown in FIG. 4, arranged within the cable screen PSC, which itself is grounded at the end and/or beginning of the power cable PCA.
At the beginning or end of the power cable detection circuitry is connected for detecting and locating a water intrusion into the cable. Such detection circuitry is for example disclosed in DE 195 44 391 A1, DE 195 27 172 and EP 0 011 754 by Pirelli Cavi e Sistemi and DE 100 19 707 A1 and DE 100 19 430 A1 of the German company Lancier. In the applications by Pirelli Cavi e Sistemi a single water sensing wire is located in each power cable of a three-phase power transmission system.
As also described in a summary article by L. Goehlich et al. in Elektrizitätswirtschaft, Heft 26, 2000, pages 1–8, the core measurement principle in such water monitoring systems is that a current source feeds a current into the water sensing wire or water sensing wires. In the normal operation condition with no water intrusion there will be no current flow between the water sensing wire and the cable screen PSC, which itself is grounded at the cable beginning or cable end.
However, during water intrusion, as illustrated in FIG. 5, water intrudes into the outer sheath PSH, the cable screen PSC and into the water sensing wire WSW such that a measurement current flows from the water sensing wire through the screen to ground. Performing such a measurement from the cable beginning as well as from the cable end allows that also the location of the failure position can be determined. For further information regarding the water monitoring process, reference is made to the above patent applications and the articles, which are herewith incorporated into the present application via reference.
As illustrated in FIG. 5, typical water sensing wires consist of a conductor WC, for example made of Cu or any other metal, and a water permeable insulation WI surrounding said conductor WC. It should be understood that conventionally the insulation WI tightly fits onto the conductor WC, however, is water permeable in order to allow the aforementioned current flow during a water intrusion. A typical diameter of the water permeable insulation and thus of the water sensing wire WSW is about 1 mm. By means of the water permeable insulation the water sensing wire is electrically insulated from the cable screen PSC in case of no water intrusion whilst in a wet condition the water sensing wire is electrically connected to the cable screen PSC in order to allow the water monitoring measurement.
However, due to the construction of the water sensing wire as shown in FIG. 5 there are certain conditions where the lifetime of a water sensing wire can be reduced. This is due to the arrangement of the water sensing wires in the screen. As shown in FIG. 6, typically the cable screen PSC is provided on the cable core (more precisely on the bedding PBE) and the cable screen PSC consists of a plurality of screen wires, which are wrapped around the bedding PBE in a stranded manner, with a pitch length of about 3 times the core diameter i.e. the screen wires PSC extend substantially parallel. The cable screen wires PSCW typically have a diameter of 0.9 mm and between the cable screen wires PSCW the water sensing wires WSW are arranged. Around this arrangement a type of conducting band PSCB is wrapped under a different wrapping pitch by comparison to the screen wires PSCW in order to contact the individual cable screen wires PSCW to each other.
For mechanical stability and electrical properties the diameter of the central conductor WC of the water sensors is only slightly smaller than the diameter of the adjacent cable screen wires PSCW. Due to the necessary water permeable insulation WI the total outer diameter of the water sensing wire is, however, slightly larger than the diameter of the adjacent screen wires PSCW. Thus, the water sensing wires WSW slightly project from the plane formed by the plurality of power cable screen wires PSCW. Therefore, obviously the conducting holding band PSCB presses onto the water sensing wires at the crossover positions PX shown in FIG. 6.
It is easily understood that during severe external mechanical influences such as deformations, temperature changes etc. the insulation WI of the water sensing wire may be unduly pressed and deformed, in particular at the positions PX, leading to mechanical and/or electrical failure of the water sensing wire. For example, a water intrusion may be detected due to a failure of the water sensing wire insulation WI by contacting the conductor WC to a screen wire PSCW leading to an incorrect detection of water intrusion. Of course, many environmental influences can cause such a reduced lifetime of the water sensing wire, because even when all conditions are appropriately set during the manufacturing of the power cable, over some time later the material of the insulation may become brittle leading to a deterioration of the insulation and consequently to mechanical and/or electrical failure.
Whilst the major impact of compressing the insulation is in the radial direction, also stresses in the longitudinal direction of the water sensing wire WSW can cause a failure. FIG. 3 shows the deformation (stretching) of a conductor made of Cu and an insulation made of polyester of a water sensing wire WSW according to the prior art ({circle around (1)}). Since the conventional combination of the Cu conductor WC and the insulation WI made of polyester, the Cu conductor WC is subjected to a plastic deformation whilst the insulation WI is still subjected to an elastic deformation when a stretching force Fo in the longitudinal direction is applied. If the force is again reduced the polyester insulation WI shrinks and bends the excess length of the plastically deformed Cu conductor WC within the polyester insulation WI. This can lead to a loop in the Cu conductor WC and this conducting loop of Cu can penetrate through the insulation WI and can thus make contact with the screen wires PSCW. This leads to fatal damage of the water sensing wire and to an incorrect detection of water intrusion.